26 THE BIOLOGY OF THE SEA-SHORE 



In the Gulf of Kiel the mean monthly temperature varies 

 between a maximum of 14° C. and a minimum of i'5° C. 

 (Semper). Great range is experienced within the tidal zone, 

 since it is liable to exposure to frost in winter and the full 

 glare of the sun in summer ; and the waters bathing it tend 

 to be similarly affected, since the more shallow the water, the 

 more readily will it be influenced by temperature variations. 



The mean annual temperature of the sea- water off the 

 western coasts of Great Britain is from 10° C. to 13° C. 

 Off Labrador it varies from 2° to 5° C. In the tropics the 

 surface temperature generally exceeds 26*7° C. Sea-water 

 freezes at —2° C. (see Johnstone, 1908 ; Fowler, 191 2 ; 

 Allingham, 1917). One of us found the surface waters 

 of the Dovey estuary to vary between 9*0° C. and I2'2^ C. 

 during November, 1913. Southern {op. cit.) gives some 

 very interesting data obtained during the Clare Island 

 Survey. For Ballynakill Harbour the result of daily obser- 

 vations taken in 2 fathoms of water (9 a.m. readings) showed 

 that the mean surface temperature is higher than the mean 

 bottom teriiperature from May to August ; lower from 

 September to March, and equal to it in April. The figures 

 for 1902 showed the highest record to be I9"6° C. on June 

 30 ; the lowest 3*30° C. on January 28, and the annual 

 range 16*3° C. The greatest difference within twenty-four 

 hours was 4*1° C. on June 30. 



Effects of Ice. — Portions of the coast are at times during 

 the winter more or less completely frozen over, and small 

 harbours may have a thick covering of ice. Sheltered 

 portions of the coast which are not exposed to surf or strong 

 tidal currents may become fringed with ice. The movement 

 of ice, along the shore and through channels, serves to 

 scrape bare the large stones and boulders wherever they 

 are exposed, so that they may be almost or entirely free from 

 algae in the spring when the ice disappears. At times, also, 

 the common barnacle {Balanus halanoides) which covers 

 their surfaces in summer may be removed. Rocks which 

 are perfectly' bare after the winter become thickly covered 

 during spring and summer with alga2 (Davis, 1913). 



